The present invention relates to improvements made to flat glass annealing lehrs.
It is known that flat glass annealing lehrs are tunnel furnaces, equipped with controlled heating and cooling means making it possible to cause the glass ribbon to follow a continuous cooling thermal cycle.
A lehr according to the current state of the art has been depicted schematically in FIGS. 1 and 2 of the appended drawings: FIG. 1 depicts the various zones that make up this lehr and the Curve of the glass annealing thermal cycle which results from it, and FIG. 2 is a section on a longitudinal vertical plane of this lehr.
Referring to FIGS. 1 and 2, it can be seen that the zones of which the lehr is composed are generally defined as follows:
Zone AO: special inlet zone for particular treatment,
Zone A: pre-annealing zone,
Zone B: annealing zone,
Zone C: post-annealing zone,
Zone D: temperate direct cooling zone,
Zone F: final direct cooling zone.
Zones A, B and C are zones in which heat exchange is by radiation and zones D and F are cooling zones in which heat exchange is by convection.
The most critical phase in the cycle of annealing the glass ribbon is in zones A and B in which the glass is in a viscoelastic state allowing the stresses generated in the glass ribbon during the operations of forming it to relax. Poor control over the cooling of the glass ribbon in these zones can give rise to temperature gradients in the glass which will generate stresses which may remain in the form of residual stresses.
Referring to FIG. 2 in which, in this lehr according to the prior art, only the heat exchangers situated above the glass ribbon 1 are depicted (the heating means and the exchangers which are situated beneath this glass ribbon are not depicted in this figure), 2 has been used to depict the system of mechanized rollers which supports and drives the glass ribbon 1 which passes through the lehr.
In zone A, a fan 4 draws in external air at ambient temperature, through a manifold 3 which supplies several groups of exchangers 5 covering the surface of the glass ribbon 1. The exchanger 5 consists of a certain number of groups of tubes of circular, rectangular or some other cross section, arranged across the width of the glass ribbon so as to vary the cooling across the width of this ribbon. The air flow rate drawn into each group of exchanger tubes 5 is regulated via a series of valves such as 6 which are installed upstream or downstream of each group of exchanger tubes 5 and the degree of opening of which allows the cooling air flow rate to be adjusted according to the target temperature set at the end of the zone A.
In zone B, a fan 7 recirculates air through several groups of exchanger tubes 8 distributed across the width of the glass ribbon 1. The temperature of the recirculated air in the groups of exchangers 8 can be adjusted by providing a hot air exhaust through the manifold 9 and by regulating some valves such as 10 and 11 which control the dilution of ambient air in the recirculated air. The air flow rate passing through each group of exchangers 8 can be adjusted via valves such as 12 which are installed upstream or downstream of each group of exchanger tubes 8 and the degree of opening of which allows the cooling air flow rate to be adjusted according to the target temperature fixed at the end of this zone B.
In zone C, a fan 14 draws in external air at ambient temperature through a manifold 17 which supplies several groups of exchanger tubes 16 covering the surface of the glass ribbon 1. The air flow rate drawn into each group of exchangers 16 is regulated via a series of valves such as 13 installed upstream or downstream of each group of exchanger tubes 16, according to the target temperature for the end of this zone C.
All the air temperatures and flow rates of air passing through the groups of exchangers in zones A, B and C are controlled by a regulating system operating each valve such as 6, 10, 11, 12 and 13, on the basis of information transmitted by temperature sensors which are installed at the end of each zone and across the width of the glass ribbon.
A study of FIG. 2 reveals that the obligation for the ducts connecting the exchanger tubes where zones A and B and where zones B and C meet to pass through the roof of the lehr, the bulk of the valves such as 6, 12, 13, and the need to provide means for compensating for the expansion of the exchanger tubes (not depicted in the figure), make it essential to separate the groups of exchangers of zones A, B and C by distances denoted by the references X and Y in FIG. 2 and known as inter-zone regions. The length of the inter-zone regions is generally of the order of 1.5 meters.
It can be seen that over the distance of the inter-zone regions x and Y, the glass strip is not subjected to the controlled radiation of the exchangers. It is therefore evident that its cooling is not controlled during the time needed for the glass ribbon to cover these inter-zone regions. In consequence, the temperature curve does not have an even profile while the glass ribbon is passing through these inter-zone regions X and Y. The glass, in these inter-zone regions, is at a temperature level which corresponds to a critical viscoelastic state. This non-uniformity of the glass ribbon cooling curve generates stresses in this glass which may remain right up to the end of cooling, in the form of residual stresses.
FIG. 3 of the appended drawings shows the temperatures of the glass ribbon and of the air passing through the exchanger tubes of zones A, B and C and over the length of the inter-zone regions X and Y. Curve 18 shows the change in glass ribbon skin temperature and curves 19, 20 and 21 shows that of the temperature of the air passing through the exchanger tubes in each zone.
The glass ribbon skin temperature (curve 18) decreases between the entries and exits of zones A, B and C:
In the case of Zone A: between points A and B;
In the case of Zone B: between points C and D;
In the case of Zone C: between points E and F.
A study of curves 19, 20 and 21 shows that the temperature of the air passing through the exchanger tubes:
In the case of Zone A: increases between points J and K;
In the case of Zone B: increases between points M and L;
In the case of Zone C: increases between points O and N.
As the inter-zone regions X and Y are not controlled, an increase in the glass ribbon skin temperature occurs therein. These increases in temperature are represented by the curve 18:
The inter-zone region for Zone A and Zone B, between the points B and C,
The inter-zone region for Zone B and Zone C, between the points D and E.
This lack of control over the heat exchange at the inter-zone regions X and Y disturbs the glass ribbon cooling curve in a range of temperatures that correspond to the viscoelastic domain, and this may give rise to the appearance of stresses, some of which will remain after total cooling, to the detriment of the quality of the glass produced.
It will be appreciated that the temperature difference of the glass in the inter-zone regions X and Y, for a given setting of the lehr, will vary according to the thickness of the glass ribbon or according to the speed at which it travels, which is dependent on the production of the line.
The present invention sets out to provide a solution to the technical problem mentioned hereinabove by eliminating the discontinuity in the glass ribbon annealing curve, which discontinuity is generated by the presence of the inter-zone regions X and Y in the lehrs according to the current state of the art, this being so as to improve appreciably the quality of the end product.
In consequence, the present invention relates to a flat glass annealing lehr equipped with controlled heating and cooling means comprising, in particular, pre-annealing, annealing, and post-annealing zones with heat exchange by radiation and temperature direct cooling zones and final direct cooling zones with heat exchange by convection, the said zones being equipped respectively,with groups of cooling-air heat exchangers situated above and/or beneath the glass ribbon, characterized in that it comprises:
a single cooling-air intake manifold for a first group of exchangers in the pre-annealing and annealing zones, which manifold is situated where the zones meet, and
a single cooling-air intake manifold for a second group of exchangers in the annealing and post-annealing zones, which manifold is situated where the zones meet.
According to the present invention, the single manifold located where the annealing and post-annealing zones meet may be produced in the form of ducts, divided vertically into two sections to which the groups of exchange of the annealing zone and those of the post-annealing zone are connected.
The present invention is also aimed at a system for controlling the temperature of the cooling air on intake to the annealing zone and on discharge from the post-annealing zone, this possibility of regulating the lehr allowing the temperatures of the cooling air passing through the groups of exchangers to be optimized, thus making it possible to obtain the ideal cooling curve for the glass ribbon passing through the lehr. This result is obtained according to the invention irrespective of the thickness of the glass ribbon and of the production of the line.
Other features and advantages of the present invention will become apparent from the description given hereinafter with reference to FIGS. 4 and 5 of the appended drawings, in which: